The development of the atrial septum in the chick under normal and experimental conditions will be investigated by light microscopy and transmission and scanning electron microscopy. Routine, histochemical and autoradiographic techniques will be used. The atrial septum will be studied from the time of its earliest appearance (day 3) through hatching. The early post-hatching septum will also be studied. Particular emphasis will be placed on the formation, development and closure of foramina secunda. These perforations in the atrial septum form late in the first week of incubation and persist until a few days after hatching. They provide one mechanism of shunting blood away from the immature lungs. The three basic components of the septum (endocardium, myocardium and extracellular matrix) will be analyzed for their individual roles in septation as well as their interrelationships. The histochemical methods will be used to localize and characterize the glycosaminoglycans and glycoproteins of the extracellular matrix. Emphasis will be placed on changes they may undergo during development of the atrial region. Autoradiography will be used to determine the mitotic activity of the cellular elements of the septum. This will help establish whether septal growth is an active or passive process. Experimental hypoplastic left heart will be used as a model system for atrial septal defects. Septae from these hearts will be analyzed similarly to the normal hearts and comparisons made of the morphological and histochemical data. This work will provide a basis for better understanding the mechanisms of normal and abnormal atrial septal development at the cellular and subcellular levels.